Are contraception effectiveness percentages per-user or per-use?

Question

The efficacy of contraception methods is typically given as two percentages: typical use, and perfect use.

For both of these, is the denominator of the percentage the number of couples who used the contraception method, or the number of instances of sexual intercourse during which it was used?

In other words, if a contraceptive is 98% effective, does that mean:

• 2% of couples who used this method (any number of times) got pregnant, or
• Of all the individual instances of sexual intercourse during which this method was used, 2% of them resulted in pregnancy?

Answer 1

Neither.

The percentages given are the percentage of couples using that method who will conceive over a year. So if 100 people use that 98% method, 2 will get pregnant each year.

Here is a page from the National Health Service in the UK that explains this as well as providing some numbers for some methods.